This invention relates to methods of hole-forming in plastic workpieces.
The method of the present invention is especially applicable to hole-forming in thin plastic material, but more particularly to the forming of lateral holes in plastic tubing of small diameter. The method is of notable advantage in the manufacture of medico-surgical items, and in particular, of laterally-ported cannulae of small diameter such as used, for example, for epidural anaesthesia.
In the latter respect, an epidural cannula is commonly formed of plastic tubing having a closed distal end and one or more lateral ports or eyes near that end opening from the tube lumen. The eyes are of small diameter and there is difficulty in forming them satisfactorily. More especially, it is present practice to form each eye by punching out a small sector of the tube-wall using a punch that impinges on the tube along a transverse path that is offset from the tube axis to an extent dependent on the cross-sectional area of eye required. There is however considerable difficulty in maintaining uniformity of eye cross-section from one cannula to another with this method, because of the problem of accurate location of the tube relative to the punch. Furthermore, there is also the considerable disadvantage that the punched-out eye is usually rough around its rim.
Epidural cannulae are introduced into the epidural cavity through a hollow metal needle having a sharp pointed tip. The needle is used to puncture the skin and underlying tissue and form a passageway to the epidural cavity, the distal end of the cannula being introduced to the cavity by pushing through the bore of the needle. The needle is subsequently withdrawn by sliding along the cannula. To ensure easy insertion of the cannula through the needle and into the cavity it is important that the rim contour of the lateral eyes in the cannula be as smooth as possible. Furthermore, there is the tendency in unskilled users to push and pull the distal end of the cannula through the introducing needle to overcome the slight frictional resistance of the cannula in the bore of the needle. This can be dangerous since the sharp end of the needle may catch on the cannula and could completely sever the tip from the rest of the cannula. The risk that the end of the needle will catch on the cannula is especially great around the rim of the lateral eyes and it is therefore important that the contour of these rims be as smooth as possible to avoid any such catching. It is important also that the rim of the eyes have a smooth contour to ensure non-irritant placement, as well as free-flow characteristics for fluid passing through the eye.